1. Field of the Invention
The present invention relates to a technology for demodulating servo data read by a head from a storage medium.
2. Description of the Related Art
In recent years, recording density has been increasing dramatically in a storage media of storage devices such as magnetic disk devices, magneto-optical disk devices, and optical disk devices. This realizes compact storage devices having extensive recording capacity, and improves the usability of the storage devices.
Such high-density storage devices use an automatic control called servo control to position a head over a target track. Signal patterns called servo data written to the storage medium are used for servo control. Servo data includes at least track numbers and servo patterns required to demodulate the head position relative to track center.
To position the head precisely over the center of the target track, head velocity control is as important as fine head position control using the head position relative to track center obtained by demodulating the servo pattern. The position of the head as well as the head moving speed can be precisely demodulated by using the servo pattern of the servo data.
Then, the current supplied to a voice coil motor that controls the rotation of the head is controlled based on the difference between the head moving speed required to move to the next target track and the head moving speed at the current track. Accordingly, the head can be positioned over a target track quickly and precisely.
However, demodulated head moving speed is estimated based on the head position obtained by the demodulated servo data, and therefore, the head moving speed is not accurate and some error is expected.
To circumvent this problem, various servo data demodulation methods are disclosed in, for example, Japanese Patent Application Laid-open Nos. H8-180620, H2-168411, 2000-123506, 2003-338145, 2003-346438, 2000-173210. According to the conventional methods, the head speed is demodulated along with the head position using signals read from the servo data. That is, the moving speed of the head is demodulated directly from the servo data and not calculated from the demodulated positional information. Thus, the head speed can be demodulated more accurately.
However, the dramatic increase in the recording density has become possible because of a decrease in the track width. The decrease in the track width has given rise to limitations in the precision by which servo patterns of the servo data used for demodulating the head speed are formed. As a result, in the conventional technologies disclosed in patent documents 1 to 6, the demodulated velocity tends to include a repeatable velocity error, which is a velocity error synchronous with the disk rotation. As a result, a feedback control performed using the incorrect demodulated velocity will be inaccurate.